Atomic frequency standards are devices characterised by very precise and accurate frequencies of operation, the basic resonance system of which is an atom or a molecule experiencing a transition between two well defined energy levels. The general principle of operation of frequency standards is described in the book "The Quantum Physics of Atomic Frequency Standards" by J. Vanier and C. Andion, published by Adam Hilger, Bristol and Philadelphia, 1989.
Embodiments of atomic frequency standards using a gas cell and a maser are described in Swiss Patent no. 640 370 and U.S. Pat. No. 4,316,153 respectively.
In order to illustrate the operation of a known atomic frequency standard, there will now be described an atomic frequency standard employing a gas cell having reference to FIG. 1 of the accompanying drawings.
The frequency standard shown schematically in FIG. 1 comprises essentially an atomic resonator 10, a crystal oscillator and associated frequency multiplier or synthetiser circuitry 11, and a feedback circuit 12. The atomic resonator 10 consists principally of a lamp 13, a filter cell 14, a microwave cavity 15, an absorption cell 16 and a photoelectric cell 17. A power supply 18 provides the energy necessary to drive the oscillator and associated circuitry 11, lamp 13 and control the temperature of the various components of the atomic resonator 10. A conventional heating coil 18a is supplied from and controlled by a power supply circuit 18.
Another power supply circuit 19 provides power to supply a magnetic field to the microwave cavity 15 via a coil 19a. Further, the microwave cavity 15 is surrounded by a magnetic shield 20b to block external magnetic fields from influencing the operation of the atomic resonator 10.
In the atomic frequency standard shown in FIG. 1, there is produced by optical pumping a population inversion between the hyperfine levels of the ground state of the atoms which are generally alkali metals such as potassium, sodium or rubidium. In the case of a frequency standard using rubidium, a standard optical pumping arrangement as will now be described is used.
The absorption cell 16 contains the isotope rubidium 87 the spectrum of which comprises the two hyperfine components A and B, and an appropriate buffer gas such as nitrogen. The absorption cell 16 is illuminated by the rubidium 87 lamp 13 through the filter cell 14 which contains a rubidium 85 vapour, the absorption spectrum of which contains the hyperfine components a and b. The components A and a exist practically in coincidence whilst the components B and b are completely separated. The component A of the emission spectrum of the lamp 13 is therefore essentially eliminated by the filter cell 14 so that the light which reaches the absorption cell 16 is predominantly constituted by light at the frequency of the component B. Only the atoms of the rubidium 87 of the absorption cell 16 situated in the lower hyperfine level (F=1) absorb light and are transported into higher states.
After the rubidium atoms in the absorption cell 16 have been thus excited, they relax to either the upper hyperfine level (F=2) or to the lower hyperfine level of the ground state by collisions with nitrogen molecules of the buffer gas. Since these atoms are immediately excited by the arrival of the light, the lower level (F=1) is depopulated to the benefit the upper level (F=2). Because of this asymmetry in the pumping light, there is thus brought about a population inversion of these two levels and the absorption cell 16 becomes practically transparent to residual radiation from the lamp 13.
The absorption cell 16 is arranged in the microwave cavity 15 which is excited by the circuitry 11 to a frequency close to 6835 MHz, which frequency corresponds to the separation energy of the hyperfine levels F=1, m.sub.f =0 and F=2, m.sub.f =0 which brings about the hyperfine transition accompanied by the emission of electromagnetic radiation between these two levels. As soon as the atoms which participate in the stimulated emission arrive at the lower hyperfine level (F=1), they are optically pumped and transported into the excited states.
During this process, the magnetic shield 20b reduces the ambiant external field to a low level, and a small, uniform, axial magnetic field is produced by the magnetic field coil 19a driven by the power supply circuit 19. The magnetic field thus produced in the absorption cell 16 displaces the energy values of the hyperfine levels according to the known Zeeman effect and therefore adjusts the exact frequency of the electromagnetic radiation emitted in the above described stimulated emission.
The greater the number of stimulated emissions, the greater will be quantity of light absorbed in the absorption cell 16 and the smaller will be the quantity of light arriving at the photoelectric cell 17. The current produced by the photoelectric cell 17 is therefore at a minimum when the frequency of the excitation signal of the microwave cavity 15 is at the transition frequency.
The quartz oscillator 21 of the circuitry 11 produces a signal at 5 MHz, which is modulated in a phase modulator 22 to a relatively low frequency (about 100 Hz to 1 kHz) produced by a low frequency generator 23. The modulated signal is applied to a multiplier/synthesizer 24 to obtain a signal having the stimulated emission frequency of 6835 MHz. It is this signal which is employed in order to excite the microwave cavity 15.
The signal furnished by the photoelectric cell 17 is received by an amplifier 25 of the feedback circuit 12, then applied to a phase comparator 26 which also receives a reference signal from the generator 23 of the circuitry 11 in a manner to bring about a synchronous detection enabling determination of whether the carrier frequency of the signal applied to the microwave cavity 15 is well centered on the hyperfine transition frequency (6835 MHz). Any shifting is indicated by an error signal at the output of the phase comparator 26. This signal is sent to an integrator 27, which is employed in order to control a Zener diode 28 coupled to the quartz oscillator 21 and which modifies the frequency of the latter so as to maintain the multiplied frequency of the quartz oscillator 21 centered onto the frequency of the hyperfine transition of the rubidium 87.
The stability and precision of the frequencies of operation of the atomic frequency standard of FIG. 1 depend upon the interaction of the atoms or molecules in the absorption cell 16 with the electromagnetic field in the microwave cavity 15 whilst the atoms or molecules is undergoing the above-mentioned stimulated emissions. The electromagnetic field in the microwave cavity 15 has essentially the same frequency and wave length as the atomic or molecular hyperfine transition radiation, and the physical size of the microwave cavity is related to the wave length of the radiation.
The stability and precision of the frequency of operation of the atomic resonator 10 also depend on good temperature control of the lamp 13, the absorption cell 16 and the filter cell 14. This is connected with the fact that the hyperfine transition frequency as interrogated by the multiplied frequency of the quartz oscillator 21 and detected by the light signal impinging upon the photoelectric cell 17 is influenced by the simultaneously occuring optical pumping process. The hyperfine transition frequency is slightly shifted depending on the spectrum and the intensity of the light absorbed, which is in turn a function of the temperatures of the lamp 13, filter 14 and the absorption cell 16. Furthermore, shifts in the hyperfine transition frequency due to collisions with the buffer gas are a function of the pressure and temperature of the rubidium 87 and buffer gas in the absorption cell 16.
In some prior art atomic resonators not requiring heating and/or temperature control of the atomic or molecular elements in the absorption cell, electrodes have been located circumferentially around the absorption cell within the microwave cavity in order to reduce the physical dimensions of the microwave cavity, and to intensify and orient the electromagnetic field in the region of the absorption cell within the microwave cavity. The resulting concentration of the electromagnetic field in the region of the absorption cell optimizes the filling factor and the quality factor of the microwave cavity resonator. The filling factor is the ratio of the total magnetic energy in the space occupied by the atomic or molecular elements in the absorption cell, to the total magnetic energy in the resonator; the higher the filling factor, the better the response of the atomic resonator. The quality factor is given by the ratio of the frequency of the considered resonant mode of the cavity to its resonance line-width and determined by the ratio of the energy stored to the power lost via the cavity.
The electrodes of such prior art resonators however are bonded to the absorption cell, and secured in position relative to each other by the use of a fixative such as an appropriate resin. The dielectric properties of the fixative used diminish the intensity and uniformity of the electromagnetic field in the region of the absorption cell. They are furthermore electrically and thermally insulated from the cavity walls and are designed according to design equations which rely upon this electrical and thermal separation.
Such electrodes also provide a thermal mass within the microwave cavity which makes accurate control of the temperature of the microwave cavity, and the absorption cell within, more difficult. The electrodes act to block the transfer of the heat from the walls of the microwave cavity to the interior of the cavity and to the absorption cell located thereat thereby reducing the thermal response time of the atomic resonator, and storing and subsequently radiating heat when it is desired to reduce the heating in the microwave cavity.
In prior art atomic resonators, it is further necessary to provide energy to heat not just the contents of the absorption cell where the temperature is important but also the other areas in the microwave cavity where accurate control of temperature is not required.
In addition, the fact that areas of the microwave cavity other that the absorption cell are being heated means that initially the warm-up time of the prior art atomic frequency standards is greater than desired.